Adhesive dispensing devices such as disclosed, for example, in U.S. Pat. Nos. 4,969,602 to Scholl; 5,065,943 to Boger et al; and, Re. 33,481 to Ziecker et al, all owned by the assignee of this invention, have been employed in a number of applications such as the manufacture of disposable diapers, incontinence pads and similar articles wherein a comparatively low adhesive application temperature and good bond strength are required using as little adhesive as possible. In dispensers of this type, a bead of adhesive is extruded from the adhesive discharge bore formed in a nozzle plate mounted to the nozzle of the dispenser. This adhesive bead is then impinged by air jets emitted from bores formed in the nozzle plate to attenuate or stretch the adhesive bead forming a thin fiber, and to then twist or swirl the fiber so that it is deposited in a spiral pattern onto the substrate. The flow of adhesive to the nozzle plate is controlled by the operation of a plunger valve carried by the adhesive dispenser which is movable between an open position and a closed position relative to the adhesive discharge bore in the nozzle plate.
The nozzle attachments or plates disclosed in U.S. Pat. Nos. 4,969,602; 5,065,943; and, Re. 33,481 satisfy many of the requirements associated with the manufacture of disposable diapers and similar articles, but certain limitations remain. One problem involves inefficient heat transfer to the adhesive as it flows from the adhesive dispenser and nozzle into the nozzle plate. Although the adhesive dispenser, and/or the manifold which supplies adhesive thereto, contain heating elements to maintain the temperature of the hot melt adhesive, no heaters are present within the nozzle portion of the dispenser. Heat transfer to the nozzle plate is therefore completely dependent on heat conduction from the dispenser body, through the nozzle and then to the nozzle plate. As a result, at least some temperature drop in the adhesive can occur in the course of its passage to the nozzle plate prior to discharge onto a substrate.
Another concern with adhesive dispensers having nozzle plates of the type disclosed in U.S. Pat. Nos. 4,969,602; 5,065,943; and, Re. 33,481 is leakage or drooling of adhesive from the nozzle plate, particularly when the adhesive dispenser is operated intermittently. As mentioned above, the adhesive dispenser includes a plunger movable with respect to the adhesive discharge bore in the nozzle plate to control the flow of adhesive thereto. In each of the nozzle plate designs disclosed in the patents mentioned above, a relatively large cavity is formed between the adhesive discharge bore in the nozzle plate and a seat within the nozzle which engages the tip of the plunger. Adhesive can pool or collect within this cavity and leak through the adhesive bore in the nozzle plate when the plunger is in a closed position. This creates stringing or drooling of adhesive which can clog the adhesive discharge bore and/or the air jet bores formed in the nozzle plate.
A still further potential concern with nozzle plates of the type described in the patents mentioned above is leakage of adhesive into the air jet bores of the nozzle plate at the point of introduction of the adhesive from the nozzle of the dispenser into the nozzle plate. In U.S. Pat. Nos. 4,969,602 and Re. 33,481, for example, a metal-to-metal seal is provided between the nozzle plate and the nozzle of the adhesive dispenser in the area between the adhesive discharge bore and air jet bores of the nozzle plate. Recognizing the potential difficulties with this type of a seal, U.S. Pat. No. 5,065,943 to Boger et al discloses a nozzle cap assembly in which an O-ring is interposed between the adhesive bore of the nozzle plate and its air jet bores so that when mounted to the nozzle of an adhesive dispenser an improved seal is created between the adhesive flow path and air flow path. While the construction disclosed in U.S. Pat. No. 5,065,943 is an improvement, the O-ring can become dislodged or lost during maintenance or cleaning of the nozzle cap, thus presenting a sealing problem when the cap is subsequently replaced.